1. Field of the Invention
This invention relates to a method for preparing a novel class of fluorinated polyorganosiloxanes. The copolymers effectively stabilize the structure of partially cured foams prepared from moisture curable RTV polyorganosiloxane compositions.
2. Background Information
Room temperature vulcanizable (RTV) polyorganosiloxane foams have been obtained by introducing a blowing agent into a one-part moisture curable RTV elastomeric composition. These types of compositions are well known, and are typically prepared by mixing together, in the absence of atmospheric moisture, at least one hydroxyl endblocked polydiorganosiloxane and at least one silane or siloxane containing three or more silicon-bonded hydrolyzable groups such as carboxyl, alkoxy, ketoximo, amido or aminoxy. The compositions can also include curing catalysts, fillers, adhesion promoters, pigments, flame retardants and other additives to modify the appearance and/or the properties of the cured elastomer.
A disadvantage of employing moisture curable RTV compositions for preparing foams is the relatively long time period required for the composition to cure to the extent that the foam becomes self supporting. Once the expansion due to the action of the blowing agent is substantially completed, the partially cured foam begins to collapse. In addition, liquid material drains from the foam and eventually forms a layer of solid rubber beneath the foam. The relatively high density of the resultant foams and the presence of appreciable amounts of solid rubber may more than offset the advantages of utilizing these foams for insulation, cushioning and other typical foam applications.
The problem of maintaining the structure of moisture curable polyorganosiloxane foams during curing has been addressed by F. Modic and B. Boudreau in U.S. Pat. No. 4,368,279, which issued on January 11, 1983. Modic and Boudreau teach maintaining the initially produced foam under a vacuum of at least 600 mm of mercury for the time required for the foam to become self supporting. The one example of a moisture curable RTV composition in this patent discloses that following vigorous stirring of the composition, ambient pressure was gradually reduced to 10 mm of mercury over a period of 2 to 5 minutes and maintained at this level for 10 minutes.
While the application of vacuum to a foam during curing may be feasible for the formation of slab stock using a foam machine, it requires specialized equipment including vacuum pumps and a substantially air-tight chamber for preparing the foam. The use of vacuum would not be practical, or in some instances even feasible, if the foam is prepared at the location where it is to be installed, which can be at relatively remote sites. In such situations, it would be far more desirable to have all of the ingredients required to prepare the foam, including a blowing agent, packaged in a single container such as an aerosol can.
Foamable one-package RTV elastomeric compositions are disclosed in German Patent Publications No. 2,909,443 (published Sept. 18, 1980) and 2,911,971 (published Oct. 4, 1980), both of which are assigned to Perrenatorwerk Alfred Hagen GmbH. The compositions include a low boiling solvent and/or a compressed gas as the blowing agent and are packaged in a flexible container that is equipped with a valve. The container forms one part of a 2-compartment pressurizable dispensing package. The second compartment contains a compressed gas that supplies the pressure required to dispense the RTV composition from the package. In the exemplified composition, enough gas diffuses through the wall of the flexible container to act as a blowing agent for the foamable composition. The composition contains 50% by weight of chalk and 4% by weight of silica. This amount of filler is required to maintain the structure of the foam during curing, however the density of the cured foam is very high due to the large amount of filler present. U.S. Pat. No. 4,229,548, which issued on Oct. 21, 1980 to Sattlegger et al. discloses RTV compositions similar to those disclosed in the foregoing German Patent Publications, but teaches using pressurizable containers equipped with nonpermeable, flexible inner containers for the foamable compositions.
The prior art discloses additives for reducing the density of polyorganosiloxane foams prepared by the reaction of polydiorganosiloxanes containing silicon-bonded hydroxyl groups with curing agents containing silicon-bonded hydrogen atoms. U.S. Pat. No. 4,026,845, which issued to Y. K. Kim et al. on May 31, 1977, teaches known fluorine-containing surfactants for this purpose. The surfactants contain fluorinated carbon atoms, and include both organic and organosilicon compounds.
U.S. Pat. No. 3,511,788, which issued to J. Keil on May 12, 1970, discloses using a foam stabilizer to prepare foams from (1) organic liquids having surface tension values in contact with air at 25.degree. C. of at least 2.2.times.10.sup.-4 newtons per centimeter, or (2) organic plastisols containing a plasticizer and a vinyl resin such as polyvinyl chloride. The foam stabilizer is an organosiloxane copolymer containing SiO.sub.4/2 units and units selected from (CH.sub.3).sub.3 SiO.sub.1/2 and Q(CH.sub.3).sub.2 SiO.sub.1/2 where Q is defined as a "solubilizing" group that makes the copolymer at least partially compatible with the organic liquid or plastisol to be foamed. The patent teaches that examples of solubilizing groups that can be employed in the Q radical include carboxyl, ester, amide, amino, mercapto, halocarbon, nitrile, nitro, carbonyl and "higher hydrocarbon groups." Fluorocarbon groups are not specifically disclosed. Because polydimethylsiloxanes typically exhibit surface tension values below 2.2.times.10.sup.-4 newtons per centimeter, using these polydimethylsiloxanes as a foamable material in combination with a foam stabilizer disclosed in the aforementioned Keil patent would be outside the scope of the invention defined in this patent.
U.S. Pat. No. 3,328,349, which issued to Charles Lentz on June 27, 1967, discloses benzene soluble copolymers consisting essentially of (C.sub.n F.sub.2n+1 CH.sub.2 CH.sub.2)(CH.sub.3).sub.2 SiO.sub.0.5 units, where n has a value of from 1 to 10, and SiO.sub.4/2 units. An average of from 0.3 to 1.0 fluorine-containing siloxane units are present per SiO.sub.4/2 unit. Lentz teaches that the disclosed copolymers are useful defoaming agents. On the basis of this teaching, copolymers of the type disclosed by Lentz would not be considered likely candidates to stabilize the structure of partially cured polyorganosiloxane foams.